Acute respiratory infection (ARI) is a major cause of morbidity and mortality. This application focuses on two of the most prominent causes of ARI, Streptococcus pneumoniae and non-typeable Haemophilus influenzae (NTHi). Colonization of their common niche, the nasopharynx, is extremely common for both species and represents the first step in their pathogenesis. Recent clinical data suggests that vaccination resulting in reduced carriage of one species may exacerbate disease caused by the other. This observation, together with preliminary data confirming that these species compete in vitro and in animal models of colonization, form the rationale for this application to determine the specific mechanisms of competition between S. pneumoniae and NTHi. The approach will focus on themes that are common in their pathogenesis. Both organisms express cell-surface phosphorylcholine (ChoP), an unusual bacterial structure that allows for binding to epithelial cells expressing the receptor for platelet activating factor (rPAF) but renders the organisms sensitive to innate immunity mediated by C-reactive protein (CRP). In addition, a neuraminidase expressed by S. pneumoniae can desialylate host receptors and the sialylated LPS of H. influenzae. Competition involving bacterial phosphorylcholine and the effects of neuraminidase will be analyzed in three highly exploratory specific aims. Aim 1 will test the hypothesis that there is direct bacterial-bacterial interaction that results in competition in their common niche, the mucosal surface of the nasopharynx. The effect of pneumococcal neuraminidase on NTHi in a chinchilla model of nasopharyngeal colonization and competition involving choline acquisition in vitro will be determined. Aim 2 examines whether bacterial interactions with respiratory tract epithelial cells promote competition in adherence involving binding via the rPAF or by desialylation of host receptors. Aim 3 explores whether the host's epithelial cell inflammatory response and the elaboration of TNFa and CRP favor adherence and/or colonization of one species over the other. The information obtained will facilitate a longer-term effort to understand the molecular mechanisms for competition between these important pathogens in their common niche. Future strategies to decrease disease burden by reducing carriage could result from a greater understanding of interactions between species. [unreadable] [unreadable]